Modern key fobs use small coin-cell batteries — usually CR2032 or CR2025. When the battery dies, the fob stops communicating with the vehicle. Doors won't unlock. Some vehicles won't even start.
Most fobs have a physical key hidden inside that can manually unlock the door. Press the release button on the fob and the metal key slides out. Use it on the door lock cylinder (sometimes hidden under a cap on the handle).
Replacing the battery takes 5 minutes. Open the fob shell with a small flat-blade screwdriver in the seam. Note battery orientation. Replace. Snap shut. Done.
Some luxury fobs (BMW, Mercedes, Audi) have unusual battery types or require dealer programming after replacement. Check the owner's manual or call the dealership before opening.
If a new battery doesn't fix it, the fob itself may have failed — water damage, dropped one too many times, or worn-out buttons. Replacement fobs from the dealer cost $100–$400 and usually require programming.